1. Field of the Invention
This invention relates to a process for the direct oxidation of propylene to propylene oxide under conditions such that propylene is above its critical temperature and pressure. Under such conditions the selectivity of propylene to propylene oxide is enhanced. The instant process does not require the use of a solvent or catalyst.
2. Brief Description of Prior Art
Propylene has been directly oxidized to propylene oxide under a variety of different conditions. This is illustrated by the following patents:
U.S. Pat. No. 1,995,991 describes the direct oxidation of olefin hydrocarbons. In Example III of the patent, propylene oxide is prepared by the reaction of propylene and oxygen at a temperature of from 270.degree.-360.degree. C. for a reaction time of 28 seconds. The patent states that the size and shape of the reaction vessel is important and that yield of product is increased by increasing the ratio of free space to surface. On page 4, lines 9 to 15 it is stated that it is preferred to operate at atmospheric pressure although pressures greater than one atmosphere can be used and 250.degree. C.
U.S. Pat. No. 2,392,316 generally describes the oxidation of hydrocarbons. The patent states that one volume of a gaseous unsaturated hydrocarbon is mixed with about 7 volumes of air and heated to a temperature of 400.degree. to 800.degree. F. while under a super-atmospheric pressure of nor more than 500 pounds per square inch.
U.S. Pat. No. 2,530,509 describes the production of propylene oxide by direct oxidation which comprises reacting at a temperature of about 275.degree. C. to about 700.degree. C. a hydrocarbon gas selected from propane and propylene with a gas containing molecular oxygen in an amount to give in the total gas mixture a molar ratio of hydrocarbon to oxygen of at least 2 and not more than 20. The gasses are stated to be confined in a reaction zone having a ratio of area of gas contacting surface to volume of free space of at least 5 square centimeters per cubic centimeter. The patent states in column 6, lines 39-43 that the reaction itself takes place in a relatively short time and space and that it is difficult if not impossible to lengthen the reaction time.
Further, it is stated in column 5, lines 48-53 that operating pressures below 200 pounds per square inch absolute obtain the maximum efficiencies and yields.
U.S. Pat. No. 2,689,253 is directed to the oxidation of hydrocarbons. Examples IV-VI describe the oxidation of propylene and propane. These examples describe the reaction of propylene or a mixture of propane and propylene with oxygen to give a vapor phase partial oxidation. The reactions are carried out at temperatures of 650.degree. F. under a pressure of about 100 pounds per square inch gauge to initiate reaction. After initiation, it is stated that the temperature rises to about 850.degree.-900.degree. F. and further reaction is halted within about 0.5 seconds by passing the reaction mixture into a cascading stream of water.
U.S. Pat. No. 3,132,156 is directed to the selective non-catalytic vapor phase oxidation of saturated aliphatic hydrocarbons to olefin oxides. This patent describes the preparation of propylene oxide. It is stated in the patent that the reaction is conducted in such a manner that a critical balance is maintained between temperature, pressure, oxygen, and contact time in a critical environment of reactants, products and temperature gradients in the reaction zone (Column 2, line 68 through line 2 of column 3). Further, the following is stated in column 8, line 73 through line 2 of column 9:
`However, it has been discovered that all the relationships previously set forth are to no avail unless the oxidation takes place at pressures about 20 to 150 p.s.i.g.`
U.S. Pat. No. 3,483,229 relates to the non-catalytic vapor phase oxidation of hydrocarbons in a dilution reactor. The examples of this patent describe the reaction of a mixture of propane and propylene with oxygen in a dilution reactor. The reaction is carried out at a temperature ranging from about 425.degree. C. to about 575.degree. C. and a pressure of from about 20 p.s.i.g. to about 150 p.s.i.g.
U.S. Pat. No. 3,625,847 is directed to the direct oxidation of propylene to propylene oxide. The patent states that a gaseous mixture of propylene together with a gas selected from air, oxygen, nitrous oxide and mixtures thereof is fed to an electrical alternating current gas discharge which is stated to be of such nature that the electrical alternating field is applied perpendicularly to the direction of flow of the gas mixture. The gas discharge is electrically in series with a dielectric of high dielectric constant and comes into contact with the gas discharge plasma of the dielectric and in which the gas mixture containing propylene-1,2-oxide continuously leaves the discharge chamber.